How to get Y value at a given X value of a trendline

Question

I have created a trend-line using the lpoly command (local polynomial smoothed trendline).

I want to find what the y value of that trend-line is at any given x value.

How can I do this?

Answer 1

One can do this using the generate() option of the lpoly command:

webuse motorcycle, clear
lpoly accel time, generate(x y)

The values are stored in the y and x variables (here showing the first 10 observations):

list y x in 1/10

     +------------------------+
     |          y           x |
     |------------------------|
  1. | -1.6245329   2.4000001 |
  2. |  -1.775922   3.5265307 |
  3. | -1.9832878   4.6530613 |
  4. | -2.2217888   5.7795918 |
  5. | -2.3814197   6.9061224 |
     |------------------------|
  6. | -2.5199665    8.032653 |
  7. | -3.3919962   9.1591836 |
  8. | -8.8572222   10.285714 |
  9. | -16.957709   11.412245 |
 10. | -26.693355   12.538775 |
     +------------------------+

If these two variables are then plotted, it can be seen that this is indeed the case:

twoway line y x